Nonbleeding blue print and process of making the same



Patented Apr. 9, 1940 UNITED STATES NONBLEEDING BLUE Pam'r AND rnocnss or MAKING THE SAME Harold J. Bmuk, Chicago, and Morris Dickason,

Oak Park, Ill., assignors to The C. F. l'ease Company, Chicago, 111., a corporation of Delaware No Drawing. Application July 29, 193': Serial No. 150,412

2 Claims.

This invention relates to photoprints and an improved process of making the same, and more particularly to a process whereby bleeding of the photoprints may be prevented.

5 The usual process of blueprinting consists of applying a light sensitive coating to paper or a like material, exposing the paper to a light with a tracing or negative between the light source and the coated side of the paper and thereafter dev veloping the coated paper to produce a blue sheet with white lines or a while sheet with blue lines.

The coating ordinarily used consists of a mixture of potassium ferricyanide and an organic salt of iron such as ferric ammonium oxalate, and is ll applied by means of an aqueous solution of the mixture. Upon exposure to light the potassium ferricyanide and the ferric ammonium oxalate are partially reduced to potassium ferrocyanide and ferrous ammonium oxalate, respectively. When the paper is washed with water the reduced chemicals react with the unreduced to produce the insoluble blue compounds known as Turnbulls blue and Prussian blue. The unexposed chemicals as well as the excess of the unreduced compounds,'being soluble, are washed away, leaving blue lines or areas in the exposedportions.

. Great difficulties, however, are encountered in attempting to produce a sharp line of demarcation between the exposed and unexposed portions.

When the water is applied to fix the exposed chemicals and wash off the soluble substances it causes the occurrence of what is known as bleeding. The unreduced chemicals of the exposed 3. portion dissolve in the water and thus react with the reduced ferricyanide and ferric salt of the same area to form the-essential insoluble blue substances. However, before this reaction is complete some of the reduced chemicals are carried 40 by the water into the unexposed portions of the paper thereby forming insoluble blue compounds on the edge of the unexposed portions of the paper, producing a ragged line or other unsightly irregularities. a 45 Various processes have been devised .to prevent bleeding but the present invention has certain advantages over the same.

One feature' of our invention is the prevention of bleeding and thus the production of photo- 50 prints having clearly defined lines. Another feature-is an increase in the speed at which the printing may be carried on. Another feature is the development of a process in which the print-;

ingmay be carried on at a constant speed res5 gardless of the concentrations-of the sensitizing agents used. Still another feature is the pro duction of a more uniform color than has been possible heretofore. Other features and advantages will be apparent from the following description of our invention. 5

We have found that pure potassium ferrlcyanide, although senstive to light when mixed with a light sensitive ferric salt, is practically completely unaffected by light when not so mixed. Certain ferric salts, such as ferric ammonium ox- 10 alate, for example, are, on the other hand, very easily reduced by light. In fact, the ferric'salts are more sensitive to light when used alone than when mixed with the ferricyanide as in the usual blue print process. Potassium ferricyanide re-JB acts with ferrous compounds (which are formed by the reduction of ferric salts) to form ferrous ferricyanide, a water-insoluble blue substance known as Turnbulls blue. Thus, if the paper to be printed is coated first with a light sensitive 20 ferric salt and then with a ferricyanide of an alkali metal and thereafter exposed to light the paper need only be washed to produce a well colored non-bleeding blue print. The ferricyanide, which is unaffected by the light, reacts with the 25 exposed reduced ferric salts to form ferrous ferricyanide (Turnbulls blue) which is insoluble and does not run or bleed.

Also, where the two layers meet there occurs, to a certain extent, a mixing of the ferricyanide and 30 the ferric salt. As a result, at this interface both compounds are at least partially reduced upon exposure. Washing with water thus causes an additional reaction, to witthe reaction between the ferrocyanide and the unreduced ferric salt to form ferric ferrocyanide (Prussian blue) which is alsoinsoluble. Prussian blue is deep blue in color and, though present only in, a small amount, serves to deepen and improve the color of the print. On the unexposed portions the fer- 40 ric salts and the ferricyanide, being water soluble, are washed away leaving a whitebackground.

The first layer of thecoating may be composed of any light sensitive ferric salt. Among the compounds which have been found particularly satisfactory are ferric ammonium oxalate, ferric ammonium citrate, ferric chloride, ferric oxalate, ferric potassium oxalate, ferric sodium oxalate, ferric ammonium chloride, ferric ammonium sulfate, and ferric ammonium tartrate.

The salt may be applied to'the paper or like material in any convenient manner. A uniform coating which adheres firmly to the paper may be obtained by applying an aqueous solution of the salt either by dipping, spraying, or by passing the paper over a rollerwhich is so mounted as to dip in the solution. method is used, it is usually desirable to use a resilient roller and rotate the roller in the opposite direction to that in which the paper is moving. roughened slightly and the solution penetrates more deeply into the surface of the paper. 3 As a result, the salt and, after the process is completed, the coloring material adheres more firmly to the surface of the paper.

The concentration of the ferric salt solution may vary over a wide range. Any quantity between a few'per'cent and saturation may be used with satisfactory results. After application the solution should be allowed the same time to soak into the paper and to dry as is used with the ordinary blue printing solutions.

The upper or second layer of the coating, which may be applied-as soon as the first layer is dry, consists of a ferricyanide compound preferably of an alkali metal such as potassium. This coating also may be applied in any convenient manner. The application of a solution of the compound is usually found to be the simplest and best method of coating. We prefer to apply this solution to the paperby means of a roller which dips into the solution. We have found that in using rollers to apply the solution it is advantageous to rotate the roller in the same direction as that of the movement of the paper. A more even coating and less contamination of the upper layer with the ferric salts of the lower layer results. Also, pin spot defects, caused by failure of the coating to cover certain small portions of the paper, are eliminated ifthis trouble is not inherent in the paper stock. If it is desired to make the paper completely bleedproof, it is usually preferable to use two successive rollers to apply the second layer. In this way a heavier but still uniform coating is obtained.

In using a roller to apply the coating to the paper, the rate of travel of the paper and the surface speed of the roller may be adjusted so that the desired amount of solution will be picked up by said roller and applied as an even coating and at a rapid rate. In the coating of blue print paper as now practiced, an excess of the coating solution is usually applied, which excess is then removed by compressed air or by a scraper.

Before the paper is rolled up for storage, the

second roller should be perfectly dry. The dry- 1 over the ordinary. blue print sensitizing agent consisting of a ferric saltand a ferricyanide,

nevertheless in such a case it will tend to cut down the color and slow up the printing. Hence this second coating should preferably be applied over a coating consisting of a ferric salt or salts alone.

The concentration of'the ferricyanide solution used should preferably-be below ten percent.

Usually it is desirable to use between one percent and six percent depending upon the concentration of ferric salt used in the first coating. We have found that a ferricyanide concentration of approximately one-fifth that of the concentration of the ferric salt is particularly satisfactory.

If, as we prefer, the last In this way the surface of the paper is- The paper containing the dry two-layer coating is exposed to light through a negative or other tracing. The light does not affect the outer coating of ferricyanide. but does reduce the exposed portions of the under coating of ferric salt, and at the interface between the two layers, where the ferricyanide and the ferric salt are mixed, both are reduced. Either sunlight or artificial light may be used to effect the reduction.

The paper undergoes a distinctive color change .as exposure takes place. From the light yellow of the unexposed sensitized paper the change is to light blue and blue grey at complete exposure quired.

The printing speed may be kept constant regardless of the concentrations of the two solutions by which the coatings are applied. This is a distinct advantage since in the ordinary process the greater the proportion of ferricyanide used in the coating, the slower is the reduction upon exposure to light and hence the slower is the printing speed which must be used.

Upon washing the exposed paper with water, the reduced ferric salts on the exposed portions are fixed by reaction with ferricyanide of the outer layer to form insoluble blue salts, while the unreduced salts on the unexposed portions, being soluble, are washed away leaving a white background. Similarly at 'the interface where both the ferricyanide and the ferric salt have been reduced, Prussian blue is formed by the reaction of the ferrocyanide and the unreduced ferric salt.

Since the ferricyanide is distributed over the ferrous salt it is dissolved by the water first and reacts to form the insoluble Tumbull's blue before the ferrous salt can be washed into the unexposed portions to cause bleeding.

The following equations represent what are probably the principal types of reaction occurring:

Potassium Ferrous Ferrous Potassium ferricyanide oxalate ferrlcganide oxalate 8KaFe(CN)l+ l( r0|)r--* )liri-6 rC|0a Potassium Ferric For: Potassium lerrocyanide oxalate ferrocyanide oxalate The ferrous ferricyanideis a water insoluble blue salt known as Tumbull's blue. It serves to color the exposed portions of the photoprint and by its precipitation prevents the running of any of the ferrous salt into the unexposed portions. The ferric ferro-cyanide, also insoluble, although present only in a small quantity aids in producing a deeper color. Thus, the unexposed portions are covered with the ferric salt and the ferricyanide. Since both of these compounds are soluble they are washed away by the water, andbleeding is prevented.

Instead of applying the two coatings before exposure it is possible to apply only the ferric salt coating before exposure and then after exposure wash the paper with a solution of-a ferricyanide to fix the reduced ferric salt. By this process a bleedproof paper is obtained." However. We Prefer to use two dry coatings before exposure since a small amount of Prussian blue. which produces a desirable color, is formed, and in general more satisfactory results are obtained in this way. Also a step in the actual printing process, the application of. a ferricyanide solution, is eliminated with a resultant saving in pi lnting time and equipment.

It is often desirable to have a very white background in the blue print. If so, certain bleaching agents may be incorporated in one of the layers ofthe coating, preferably the outer or ferricyanide layer, when the paper is being sensltized. Among the substances which are sultable for this purpose are alkali metal oxalates.

tartrates, andphosphates.

Although our invention has been described in connection with so called blue prints, it may be used as well with any photoprinting process on a fibrous sheet or on any other, suitable material which will serve the same mn'pose. Also, although certain reactions and equations have been used to illustrate the invention, we do not wish to be limited thereto.

We claim: 1. A method of making nonbleeding photoprints which comprises applyingto paper, with a resilient roller, a solution of ferric oxalate, the roller being rotated in the .direction opposite to the movement of the paper, allowing said solution to dry, and then app yin with at least one roller a solution of potassium ferricyanide to form a second coating, the roller being rotated in. the direction of the movement of the paper, exposing portions of the coated paper to light to reduce the ferric compound on these porment of the sheet material. allowing said coating to dry. then applying with a roller a solution of a ferricyanide of an'alkali metal to form a second coating, said last mentioned roller being rotated in the direction of the movement of the sheet material, and allowing the second coating to dry.

1 HAROLD J. DRUNK.

MORRIS DICKASON. 

